In the construction of glass optical fibers, a coating or coating layers are usually applied to the glass optical fibers immediately after drawing to protect the glass surface from the detrimental effects of chemical and/or mechanical attack which would otherwise occur. These coating layers are generally formed from organic UV curable oligomers, reactive diluents, photoinitiators, stabilizers, and a silane coupling agent.
Typically, UV coatings used in telecommunications are based on acrylated urethane oligomers. Coatings made from acrylated urethane oligomers can withstand a maximum continuous temperature of about 125.degree. C.
Other applications, such as medical, aerospace, and industrial applications require a higher temperature resistance, such as up to 200.degree. C. of continuous service.
Coatings based on polyimide are currently used for many high temperature applications, examples of which are disclosed in "Characterization of Polyimide Coated Optical Fibers", Optical Engineering, Jun. 1991, Vol. 30, No. 6, P. 772. However, polyimide coatings have the following disadvantages. The liquid coating composition only has a very limited shelf life, the curing must be conducted thermally which is very slow, and there is a solvent emission during curing.
Optical glass fiber coatings containing epoxy silicones are known. However, their use in high temperature environments has not been reported.
Examples of glass optical fiber coating compositions based on epoxy silicones are disclosed in Canadian Patent No. 1256821, and U.S. Pat. Nos. 4,977,198, 4,990,546, 5,057,358, 5,075,154, 5,187,251, 5,204,433, 5,260,349 and 5,381,504.
In addition to the epoxy silicone monomers, these conventional coating compositions typically contain a reactive polymer having epoxy or vinyl functional groups which can crosslink via actinic radiation. In particular, U.S. Pat. No. 5,240,971 discloses the addition of a linear polyether block having vinyl ether functional groups at both ends which polymerize with the polyorganosiloxane to produce epoxysilicone-polyether linear block copolymers. The addition of the polyether block makes the composition more miscible with polar molecules, and the coatings produced therefrom are more flexible and elastic than coatings derived from UV-cured, linear epoxy-silicones without polyether blocks (column 10, lines 46-59).
U.S. Pat. No. 5,340,989 discloses at column 3, lines 60-65 that the coating composition can further contain adherence modulators which are linear resins bearing vinyl, epoxy, vinyl ether, alcohol and the like functional groups. This patent teaches that the adherence modulator prevents adhesion of metal sheets, glass, plastics or paper to other materials they would otherwise adhere to. See column 4, lines 5-12.
A disadvantage of the above conventional coating compositions based on epoxy silicones is that specific oligomers must be synthesized to provide appropriate liquid properties, curing properties, and cured coating properties.
Furthermore, conventional coating compositions containing epoxy silicone monomers have many problems with long term stability of the composition and a viscosity which is too low. A low viscosity makes it very difficult to coat a glass optical fiber. Moreover, these conventional coating compositions exhibit problems with surface defects, such as pitting, when applied to glass optical fibers.
There is a need for a high temperature coating composition which avoids these problems.